1. Field of the Invention
This invention relates to a digital semi-active laser receiver (“digital receiver”) and more particularly to the acquisition and tracking of a laser designated pulse sequence reflected off of a target object.
2. Description of the Related Art
Certain aircraft, missiles and smart projectiles include a receiver that allows the platform to receive and process electromagnetic (EM) pulses, typically in the near IR, transmitted by a known source at a pulse repetition interval (PRI) and returned off of a target object. A typical source may generate an EM pulse that is a few nanoseconds wide with a PRI in the tens of milliseconds. The receiver may be fixed to look along the line-of-sight (LOS) of the platform or gimbaled to look along a receiver LOS relative to the platform LOS. If the source is remotely located the system is referred to as ‘semi-active’ whereas if the source is co-located on the platform with the receiver the system is referred to as ‘active’.
A core capability required to support receiver functionality is to reliably and accurately detect the presence of EM pulses returned from a real target from amidst natural clutter, a second valid designation of a different target object and countermeasures such as active jamming and to acquire and maintain a “track” at or near the source PRI to measure the line-of-sight (LOS) from the platform to the target object. Errors in acquisition and maintenance of the “track” can cause mission failure.
In a mixed analog/digital architecture, a mixture of analog and digital circuitry is used to qualify, detect and correlate individual pulses to acquire and maintain a track at or near the source PRI. The pulse qualification and detection step occurs continuously as the pulse signal comes into a filtering circuit. Analog electronics set a detection threshold, fixed or time dependent, based on previous detection amplitudes. The sequence of filtered pulses is supplied to digital circuitry that, for example, implements an N of M pulse correlation (hereinafter referred to as “correlation”) at the source PRI. Pulse selection logic (e.g. maximum amplitude pulse, last pulse logic (LPL)) is then applied to each PRI gate independently to select the pulse that best matches the rule based decision criteria. The sequence of correctly coded temporal pulses selected within each period forms the track. The receiver gain is optimized for the track. A LOS is computed for each pulse in the track. Guidance applies a filter to the track LOS values to remove occasional incorrect pulses.
As shown in FIG. 1, a digital receiver 50 includes a quad detector 52 that detects incident radiation (e.g. transmitted pulses reflected off a target or other objects, pulses transmitted to actively jam the receiver or other designated targets) and generates analog signals 53, a bank of pre-amps 54 that amplify the analog signals 53 and A/D converters 55 that convert analog signals 53 to digital signals 56 on individual channels A, B, C and D. The digital signals comprise a sequence of samples each having a sample or time index and amplitude. The samples may be referred to in context as “samples”, “time samples” or “amplitude samples” depending on whether the relevant property is the time-index or the amplitude.
A digital processor 57 detects “pulses” from the samples, establishes a “track” from the pulses, calculates a LOS to a target object for each pulse and performs additional processing to generate a track report 58 that is passed on to a command and guidance processor 60 via digital bus 62 to guide the platform to the target object. The digital processor provides a gain to optimize performance for the track. This figure illustrates that all of the pulse detection, tracking and LOS processes, except for the front-end A/D conversion, is digital and may be implemented in a single processor 57 that includes memory. The all-digital architecture is smaller, lighter weight, less expensive, uses less power and is more reliable than the mixed analog/digital architecture.
For throughput reasons, the digital receiver detects, correlates and qualifies individual pulses to acquire and maintain a track at or near the source PRI. The digital receiver detects pulses using a noise-based threshold and then applies a comb filter to remove high frequency signals (e.g. noise signals). In an “Acquisition” mode, the digital receiver applies a correlation to the pulses output by the firmware and comb filter. Assuming a correlation is found to establish a track, the receiver may use order based pulse selection logic to increase the likelihood that the acquired track is the track for the target object and not clutter in front of or behind the target object or a false track generated by clutter or countermeasures. In a “Track” mode, the digital receiver uses the phase information of the pulses in the acquired track to establish a next PRI gate. The gates are typically a few to a few tens of microseconds wide and spaced at the PRI. The digital receiver applies a threshold, fixed or time dependent, to the pulses within the gate and then applies selection logic (e.g. maximum amplitude pulse, last pulse logic (LPL)) to the thresholded pulses to identify the candidate pulse and associate the pulse with the track. The processor calculates a gain optimized for the track. Once the digital receiver enters “Track” mode, the correlation is disabled.